I don't know if this an oversight or not, but I set Weapon Strength to its weakest setting and Koopas and Goombas still take one hit to kill. I was thinking (and hoping) it would take at least two fireballs to kill them.

I tested it on very weak, Lakitu jumps from 1 hit to 2 hits, Bowser jumps from 5 hits to 8 hits, and Paratroopas lose their wings. That does mean normal koopas are still one hit. Everything else is unaffected.